This epidemiologic study will use data from the Child Health and Development Studies (CHDS), a large longitudinal study of women and their children begun in the early 1960's. A random sample of 400 children from this parent project will be selected, and a retrospective cohort design employed using stored maternal serum collected during pregnancy, prenatal and early childhood medical and lifestyle factors from interviews and records, and standardized special developmental examinations. Specific PCB congeners and toxic equivalences (TEQ's) of subclasses of PCBs will be evaluated for associations with concurrent maternal thyroid levels and throxine binding globulin, and with measurements of growth and development made on the children at 5 years of age. Cognitive development, hearing, and speech articulation will be the primary outcomes examined; secondary analyses will address vision, physical growth, and general health indicators.